It seems a little odd to me that the Persians weren't in Civilization 6 right from the start, but better late than never, and today 2K announced that the Achaemenid (Persian) Empire, with Cyrus the Great at its head, will be added to the game in an upcoming Civilization and Scenario pack.
The Persian unique unit is the famed Immortal, a replacement for the Swordsman unit, which boasts a ranged attack and strong defense in combat, while its unique improvement, the Pairidaeza—Persian Gardens—provides culture, gold, and appeal, with bonuses for adjacent tiles. I won't even try to spell its unique ability (related to satraps, as best I can make out), but it gives Persia a free trade route, and bonuses to internal trade routes, when Political Philosophy is unlocked.
As for Cyrus, he brings the Fall of Babylon ability to the table, which provides bonus movement to Persian units following the declaration of a surprise war. He also suffers reduced diplomatic penalties for declaring surprise wars, a handy trait if you're the sort of ruler who gets along well with others but really can't be trusted.
2K said Persia "can be very successful with sneak attacks," but it also used an image of Tomyris of Scythia when talking about Cyrus' military acumen, and she, you may recall, reportedly cut the man's head off and dunked it in a bucket of blood when he messed with her. Kind of mixed signals on that point, then. On the other hand, Persia can also be grown into a wealthy and powerful nation through the more peaceful application of Wonders and the Pairidaeza, so maybe that's a better way to go.
A release date hasn't been announced, but 2K said Persia will be one of two nations included in an upcoming Civilization and Scenario Pack. Based on previous releases, you can expect it to set you back $5, and it will be free if you own the Civ 6 Digital Deluxe edition.